The invention relates generally to electronic power conversion and more particularly to a quasi-AC, photovoltaic (PV) module for an unfolder PV inverter.
Photovoltaic (PV) cells generate direct current (DC) power with the level of DC current being dependent on solar irradiation and the level of DC voltage inversely dependent on temperature. When alternating current (AC) power is desired, an inverter is used to convert the DC energy into AC energy. Typical PV inverters employ two stages for power processing with the first stage configured for providing a constant DC voltage and the second stage configured for converting the constant DC voltage to AC current. Often, the first stage includes a boost converter, and the second stage includes a single-phase or three-phase inverter system. The efficiency of the two-stage inverter is an important parameter affecting PV system performance and is a product of the individual stage efficiencies.
Single phase photovoltaic inverters generally require a two-stage conversion power circuit to convert the varying DC voltage of a PV array to the fixed frequency AC-voltage of the grid. Traditional PV inverters use a DC link as the intermediate energy storage step, which means that the converter first converts the PV array voltage to a stable DC voltage then to a current that can be injected into the grid.
Traditional single phase PV inverters also undesirably control the power circuits with a fixed switching frequency using a plurality of switching devices that contribute to the overall switching losses. Switching losses are typically kept as low as possible when using traditional PV inverters by keeping the switching frequency low.
A photovoltaic generator can include many PV modules that are connected in series and parallel to form a solar generator. PV modules can lose their ability to produce power due to shading effects caused by tall objects, leaves, dust, snow, and so on. A photovoltaic generator delivers its maximum power by selecting a proper operational voltage maximum power point. Maximum power point tracking however, works well only under optimum, non-shaded conditions. As soon as parts of a PV module (one cell is enough) are covered by snow, dust, leaves, and the like, the PV generator can lose a significant portion its power.
It would be both advantageous and beneficial to provide a residential photovoltaic (PV) energy system that is easier to install, is less expensive, and has a higher efficiency than that associated with traditional PV inverters. It would be further advantageous if the PV energy system could operate in the absence of a DC disconnect mechanism. It would be further advantageous if the PV energy system could be configured to allow each PV module to operate at a corresponding operational point that is dependent upon its shading conditions.
It would be further advantageous if modules of various powers could effortlessly be combined in a system and if each quasi AC module were backward compatible when presented with a DC load voltage, further making it suitable for three-phase systems that draw constant power.
It would be further advantageous if each quasi AC module were controllable by using simple off the shelf unity power factor regulator ICs, ensuring simplicity.
Additional advantages of such a PV energy system would include, without limitation, the ability to mix different power modules in a system and also thereby maximize available roof area for energy generation, an efficiency gain over the AC module concept, no voltage on the output during installation making is safer than convention systems to install, provision of an array that can be mounted multi-directional e.g. on an igloo, and mini PV cell converters in a module, each producing pulsing current in which various configurations are possible e.g. one converter per 4 cells etc.